Gate of Heaven All Inclusive tickets (Porta del Cielo)


New member
I will be in Siena only for a day, and therefore I want to make sure that I can access the Duomo!
I was interested in buying tickets online (Gate of Heaven all inclusive).

Unfortunately, the site does not allow online purchases (?)
I did get however a link to an website in Italian only to reserve a time for the OPASiPass...

Does that mean that I have to reserve a time under OPA Si Pass... and then get my Gate of Heaven tickets at the door?

I don't understand how ticketing works.
Please help me!

Tante grazie!